Conventional plastic films such as cellophane, polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), nylon, and polyethylene terephthalate (PET) films have been widely employed for packaging. However, they are not completely satisfactory in terms of their performance characteristics.
For example, cellophane and polyvinyl chloride films generate toxic pollutants during the manufacturing and incinerating processes, respectively, and polyethylene films have been employed only for low-grade packaging materials due to their relatively poor heat-resistance and mechanical properties. Polypropylene, nylon and polyethylene terephthalate films, on the other hand, have satisfactory mechanical properties due to their relatively stable molecular structures, but generate wastes that are not biodegradable, and therefore, when subjected to a landfill without any separate treatment, they cause a significant soil pollution.
In order to solve such problems, there have been employed biodegradable aliphatic polyesters, particularly polylactic acid films. Such polylactic acid films have a good mechanical property, but represent a reduced flexibility due to their inherent crystal structures, which results in limitation to their use.
For the purpose of overcoming the aforementioned problem of the polylactic acid films, for example, Japanese Laid-open Patent Publication No. 2006-272712 discloses a method for preparing a biodegradable film composed of only a biodegradable aliphatic polyester resin except for polylactic acid. However, this method suffers from some problems in that the resulting sheet obtained during the manufacturing process is hardly subjected to a biaxial orientation due to its too low glass transition temperature (Tg), and the final film disadvantageously exhibits low mechanical strength and high heat shrinkage.
In addition, Japanese Laid-open Patent Publication No. 2003-160202 discloses that flexibility and heat sealing properties of a biodegradable film can be enhanced by blending polylactic acid with aliphatic-aromatic copolymerized polyesters. However, the transparency of the film thus obtained becomes significantly low due to poor compatibility between polylactic acid and the aliphatic-aromatic copolymerized polyesters as well as use of a plasticizer, and thus, the film is not satisfactory for packaging requiring transparency.